1. Field of the Invention
This invention relates to the production of artificial graphite and more particularly to a new and improved method and apparatus for graphitizing carbon bodies to artificial graphite.
2. Description of the Prior Art
Artificial graphite is made by heating amorphous carbon at high temperature converting the carbon to graphite which is a crystalline form of carbon. The source of carbon may be coal, but is preferably derived from petroleum in the form of petroleum coke which carbon after preheating to a temperature of above about 700.degree. C to 1200.degree. C up to about 2000.degree. C is then further heated above about 2200.degree. C to a temperature of nearly 3000.degree. C and retained at the high temperature for a sufficient time for the formation of the hexagonal crystal pattern. The thermal conductivity of graphite is higher than carbon and this combined with a low coefficient of thermal expansion gives it high resistance to shock. Although carbon and graphite are extensively used in electrical applications, graphite, despite its high cost, is usually selected as anodes for electrical applications because of higher purity, higher electrical conductivity, greater ease of machining and high oxidation resistance.
Discontinuously operated furnaces for graphitization of carbon bodies are known in the art as illustrated by the Acheson furnace and consist basically of two graphite electrodes which are disposed in the end walls of the furnace, between which electrodes the bodies to be graphitized are layered between granular resistance and isolation material. In order to heat the carbon bodies, the graphite electrodes which are short-circuited by the furnace content, are electrically connected to a current source, whereby the generated Joule's heat causes a rise of the furnace temperature to 2200.degree. up to approximately 3000.degree. C. Depending on the size of the furnace, the period of time required to heat up the furnace is approximately 1 - 3 days and the following cooling period is approximately 5 - 12 days.
The Acheson process has serious disadvantages: The operating cycle of the furnace involving long heating and longer cooling time periods retards and complicates the flow of material, particularly with respect to the great effort required for charging and discharging the furnaces. The large amounts of resistance and isolation materials used in the process require special transport as well as sorting and cleaning devices. The carbon bodies have to be stratified with great care to avoid resistance losses, without fully eliminating localized resistance variations and the resulting temperature peaks during the heating up period which deteriorate the quality of the graphite bodies. Finally, the efficiency of the Acheson method is comparatively low because in addition to the graphite bodies, a large quantity of resistance material is also heated up to the graphitization temperature and only a small part of the thermal energy supplied is recovered. Further disadvantages of this method are the difficulties to contain and discharge the poisonous gases which are generated during graphitization, for example, sulfur dioxide and carbon monoxide. Also resistor material becomes attached to the graphite bodies and the former has to be removed by grinding or other mechanized operations.
A continuous graphitization method has been suggested whereby the carbon bodies are continuously moved through a furnace which is provided with a heating device and the required energy is indirectly transferred to the bodies by heat radiation or inductive coupling. U.S. Pat. No. 1,884,600 discloses a graphitization furnace with a preheating zone heated by induction heat, a graphitization zone in which are disposed one or several induction coils and a cooling zone. Japanese Pat. No. 53 882 164 discloses a graphitization method in which the carbon body is pushed through a graphite tube heated by direct resistance heating to the graphitization temperature with an inert gas passing through the tube in counter flow to the carbon body.
The known continuous graphitization method have some advantages over the Acheson method, i.e. lesser requirements for personnel and energy, improved material handling and fewer quality variations of the produced graphite. The disadvantages are material and are in the case of induction heating the matching of diameters, respectively shape of the coils to the diameters and cross-sections of the carbon bodies, which matching is required for suitable induction coupling and also the difficulty of protecting the induction coils against overheating and attack from corrosion causing gases. As a result the graphitization of carbon bodies of different cross-sections and shapes is made quite difficult and sometimes even impossible. In some methods wherein the bodies are graphitized with indirect resistance heating, the heating tube has a limited lifetime due to evaporation because of the relative high evaporation pressure under graphitization conditions. Finally, in furnaces with horizontally arranged heating channels, particularly used for graphitization of larger carbon bodies, rollers or similar means to reduce sliding friction and wear of the channel walls must be provided which latter means have a high failure rate at the high graphitization temperature thereby greatly reducing the efficiency of continuously operated furnaces constructed according to the known state of the art.